This invention relates to blades.
The invention is more particularly concerned with rotating aerofoil blades such as fan blades used in turbomachinery, such as in gas-turbine engines.
Conventional gas-turbine engine blades are made of metal, such as titanium. These blades are capable of withstanding the temperatures to which they are exposed and have a relatively good chance of surviving impact with foreign bodies, such as bird strikes, without seriously affecting engine performance. Metal blades, however, are relatively heavy so increase the overall weight of the engine and reduce its performance. Efforts have been made, therefore, in recent years to develop blades made of alternative, lighter materials such as composite materials, for example, carbon fiber epoxy composites. The problem with such composite blades is that they are not as robust as metal blades and are more easily damaged by contact with foreign objects. Examples of composite blades are described in, EP0735161, GB646636 and U.S. Pat. No. 5,881,972. Attempts have been made to protect the leading edge of the blades, which are most likely to be subject to damage, by means of metal guards. Even with a guard on the leading edge of the blades, conventional composite blades still do not have sufficient strength to survive impact with a large, fast object, such as a bird ingested into an aircraft engine travelling at high speed.
In order to maximize efficiency of turbomachinery, the gap between the tip of the blade and the surrounding components is made as small as possible. However, the gap must be sufficient to accommodate any extension of the blades when the machinery is running at high speed. When highly twisted blades are run at high speed, up to about 3500 rpm, the centrifugal force on the blades tends to make them untwist, thereby causing them to extend in length. This leads to design problems in how to accommodate blade extension without compromising efficiency.